The invention relates to a power steering apparatus, in particular, to a power steering apparatus including a control valve of the rotary type which controls the supply to or discharge from a power cylinder of a hydraulic fluid.
A control valve of the rotary type is formed, for example, of a valve rotor which is integrally formed on an input shaft which is disposed on the steering wheel side, and a valve sleeve which is integrally formed on an output shaft adapted to be connected to the steerable road wheel and rotatable around the outer periphery of the valve rotor. In operation, the valve controls the direction of flow of hydraulic fluid to or from the power cylinder in accordance with the direction of a relative angular displacement, from a neutral position, between the input and the output shaft or between the valve rotor and the valve sleeve. The usual practice has been to provide a torsion bar interposed between the input and the output shafts to connect them together so that both the valve rotor and the valve sleeve can be maintained in their neutral position by the resilience of the torsion bar whenever the valve is non-operative. However, the neutral position of the valve rotor and the sleeve must be established when the torsional stress in the torsion bar remains zero in either direction. This makes it impossible to apply a bias load or preload across the input and the output shaft by utilizing the torsion bar.
To overcome this difficulty, there is proposed a power steering apparatus including a substantially C-shaped spring formed of a ring-shaped spring material a part of which is removed to define a notch, in addition to or in place of the torsion bar so that projections integral with the input and the output shafts are simultaneously held between the end faces which define the notch in the C-shaped spring with a given magnitude of resilience, thus allowing both shafts to be maintained in their neutral position while allowing them to be simultaneously preloaded. However, when such a C-shaped spring is used in a power steering apparatus including a control valve of the rotary type, there results an increased complexity and size in the location where the C-shaped spring is contained inside a valve sleeve integral with the output shaft, inasmuch as it is generally desirable that the projections be formed adjacent to the abutting ends of the input and the output shafts due to considerations of strength, and that the valve sleeve internally receive the input shaft and the valve rotor therein.
In addition, the abutment of the end faces which define the notch in the C-shaped spring against the projections from the input and output shafts in order to maintain them in their neutral position requires a generally increased size of the spring itself if the spring has a uniform thickness throughout its circumference as in the prior art. This is because the value of the thickness must be determined in consideration of the magnitude of the bending moment which results because of the reaction from the projections, which reaches its maximum at the central portion, that is, the region of the spring located diametrically opposite to the notch formed therein. Thus, the stress in a region remote from the central portion is reduced, which means that the thickness of such a region is not effectively utilized. In other words, the configuration of the C-shaped spring also contributes to the increased size of the arrangement around the valve sleeve.